Measurement of the refractive index of fluids, and especially the changes of said refractive indices when various substances are dissolved in said fluids at different concentrations, is an important requirement for a variety of physical determinations. For example, the determinations of molecular weights of dissolved molecules by light scattering techniques requires the measurement of dn/dc for the dissolved molecules. This differential quantity represents the change of refractive index dn of the solvent with a change of concentration dc of the solute comprised of the molecules to be measured. In the area of liquid chromatography, the concentration of dissolved molecules that have been separated by appropriate columns is determined often by means of a refractive index detector. Such an instrument responds to the changes in fluid refractive index caused by changes in concentration of the solute. These refraction index detection instruments permit thereby the monitoring of solute concentration, an important determination for many forms of liquid chromatography. Another area of application of refractive index determination relates to measurement of sugar content of various fluids. Instruments make these measurement based on a determination of the difference of left and right refractive indices of the solution whose sugar content is to be determined. For example, the Swedish company Optilab manufactures such a unit under the Tecator brand name. This technique is described in detail in the U.S. Pat. No. 4,229,105 by Silverbage.
A variety of other techniques for measuring refractive index differences have been developed over the years. Some significant prior art includes the following:
U.S. Pat. No. 4,660,974 by Machler et al. describes the determination of the spectral characteristics of the refractive index of a fluid using white light illumination and examination of the resulting interfering component beams of light. The patent also contains numerous references to other types of interferometric methods.
U.S. Pat. No. 4,569,590 by Karny et al. describes a method of creating a reference Moire pattern with respect to a cell containing a reference fluid. Upon changing the fluid, the pattern changes with said changes being used to determine the refractive index of the changed fluid.
The U.S. Pat. No. 3,619,068 by Broerman describes a refractometer of particular importance for the field of liquid chromatography. It is comprised of two intersecting channels one of which carries the fluid whose refractive index is to be measured. The second channel intersects the first at an angle other than 90.degree.. At the points of intersection the two sections of the second channel are sealed with polished transparent windows. One section contains a light source, the other a light detector. The light detector arm is filled with optical rods whose faces closest to the fluid are oriented so that the light transmitted through the fluid strikes the rods at approximately the critical angle of the fluid. As the refractive index of the fluid changes, the amount of light reaching the detector will change, permitting thereby the deduction of said fluid refractive index change.